Significant research has been undertaken in the area of lean-burn engines, where the A/F ratio is higher than stoichiometry, in order to improve fuel economy. One approach to treating such exhaust gasses involves NOx absorbents, i.e., materials which are able to absorb nitrogen oxides from the exhaust gas during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. The widely held mechanism for this absorption phenomena is that during lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the trapping material, e.g., the barium. In the regeneration mode, under a stoichiometric or rich environment, the nitrate decomposes and the NOx released is reduced catalytically over the platinum with reducing species like HC or CO in the exhaust gas.
Such conventional absorbent materials have a serious deficiency in that the barium reacts with sulfur oxides generally present in exhaust gas to form barium sulfate. This inactivates the barium for NOx absorption. It is suggested that to decompose the barium sulfate it should be subjected to elevated temperatures of at least 600.degree. C. or more in reducing gas conditions. One negative aspect of this regeneration process is that it leads to detrimental changes in the NOx absorbent such as reduced surface area and crystallization of the aluminate phases thereby reducing the efficiency of the NOx absorbent. It is also difficult to carry out on-board a vehicle and the required rich operating conditions results in emission of CO and HC. Alkali metals like potassium have also been suggested as NOx absorbents, however, they are even more easily deactivated by sulfur than alkaline earth metals like barium.
It would be desirable if a NOx absorbent could be found which would be more resistant to sulfur poisoning while being a good NOx absorbent during leanburn engine operation. The present invention overcomes the deficiencies of prior art materials. The material and method of making are disclosed herein.